In a transmission device used in wireless communication, in many cases, the PA (power amplifier) for amplifying the RF (Radio Frequency) signal to be transmitted consumes largest amount of power. Thus, in the development of transmission devices, reduction of the power consumption of the power amplifier, in other words, improvement of the power efficiency of the power amplifier, is an important task. In a recent wireless communication system, a digital modulation system such as QPSK (Quadrature Phase Shift Keying) or multi-valued QAM (Quadrature Amplitude Modulation) is employed to improve spectrum efficiency. In such a digital modulation system, to transfer data without any errors, high linearity is required of the input/output characteristics of the power amplifier.
Thus, in the power amplifier, to maintain the linearity of the input/output characteristics, average output power is set so that instantaneous maximum output (peak) power can be equal to or lower than saturated output power. In other words, in the power amplifier, since the ratio of the peak power and the average power of the RF signal to be amplified (Peak-to-Average Power Ratio, referred to as PAR hereinafter) is larger, the average output power must be set lower than the saturated output power.
In general, however, in the power amplifier, since the average output power is set lower than the saturated output power, the ratio of power acquired from the power amplifier to power supplied to the power amplifier, in other words, power efficiency, is lower. The reduction of the power efficiency of the power amplifier increases the power consumption of the transmission device, and consequently recent market demand for reduced of power consumption has not been satisfied.
Normally, the PAR of the RF signal has a unique value for each wireless communication standard. In a communication system such as CDMA (Code Division Multiple Access) or LTE (Long Term Evolution) employed in a recent mobile communication system, or a wireless communication standard employed in WLAN (Wireless Local Area Network) or terrestrial digital broadcasting, the PAR of the RF signal is set to a large value from several dB to several tens of dB.
The large PAR of the RF signal to be amplified causes great reduction of the power efficiency of the power amplifier. Consequently, the power consumption of the entire transmission device including the power amplifier increases.
As a technology for improving power efficiency in the power amplifier of the low average output power, an ET (Envelope Tracking) system is known. The configuration example of the ET system is described in, for example, Non-patent Document 1.
FIG. 1 is a block diagram illustrating the configuration example of the power amplifier based on the ET system.
As illustrated in FIG. 1, the power amplifier of the ET system includes amplifier 21 and power modulator 22. RF modulation signal 26 having amplitude and phase components is input to amplifier 21. Power modulator 22 modulates a power supply voltage VDD by using the amplitude component of RF modulation signal 26 as control signal 27, and supplies the modulated voltage to the power terminal of amplifier 21.
Amplifier 21 amplifies RF modulation signal 26 including the amplitude and phase components by using the output voltage of power modulator 22 as a power source, and outputs amplified RF modulation signal 28 via matching circuit 23.
In the power amplifier of the ET system illustrated in FIG. 1, when RF modulation signal 28 is at low output power, power modulator 22 reduces the power supply voltage supplied to amplifier 21, and thus power supplied to amplifier 21 is controlled to a necessary minimum to reduce the power consumption of amplifier 21.
As another technology for improving power efficiency in the power amplifier of the low average output power, there is a Doherty-type power amplifier.
FIG. 2 is a block diagram illustrating the configuration example of the Doherty-type power amplifier.
As illustrated in FIG. 2, the Doherty-type power amplifier includes carrier amplifier 31a that operates in class AB or B and peak amplifier 31b that operates in class C, and carrier and peak amplifiers 31a and 31b are connected in parallel.
RF modulation signal 36 having amplitude and phase components is divided to each of carrier and peak amplifiers 31a and 31b by power divider 35. At this time, RF modulation signal 36 is input from power divider 35 to peak amplifier 31b via transmission line 34b. 
First matching circuit 33a and transmission line 34a are connected in series to the output end of carrier amplifier 31a, and second matching circuit 33b is connected to the output end of peak amplifier 31b. Transmission lines 34a and 34b have lengths X/4 (X corresponding to wavelength of carrier frequency) and characteristic impedance Z0. The output ends of transmission line 34a and second matching circuit 33b are connected to each other, and a signal (RF modulation signal 38) combining the output signals of carrier and peak amplifiers 31a and 31b is supplied to load RL.
In the Doherty-type power amplifier illustrated in FIG. 2, when the amplitude r of RF modulation signal 38 is small (r≦½·Vmax:Vmax is maximum amplitude), only carrier amplifier 31a is operated, and power efficiency is largest, theoretically 78.5%, at r=½·Vmax. At this time, the impedance of a load (RL=Z0/2) seen from first matching circuit 33a is 2Z0 because transmission line 34a operates as an impedance transformer. Thus, the output power of the Doherty-type power amplifier illustrated in FIG. 2 is VDD2/4Z0=0.5 Pmax (Pmax=VDD2/2Z0:VDD is power supply voltage of carrier amplifier 31a). First matching circuit 33a matches the impedance 2Z0 of the load and the output impedance of carrier amplifier 31a with each other.
On the other hand, when the amplitude r of RF modulation signal 38 exceeds ½·Vmax, carrier and peak amplifiers 31a and 31b are both operated. In other words, at ½·Vmax<r≦Vmax, since power is also supplied to the load from peak amplifier 31b, the impedance of the load changes from Z0/2 to Z0 due to a load pull effect. Further, since transmission line 34a operates as the impedance transformer, the impedance of the load seen from first matching circuit 33a also changes from 2Z0 to Z0. Accordingly, in the Doherty-type power amplifier illustrated in FIG. 2, at r=Vmax, a power output is maximum Pmax (=VDD2/2Z0).
First matching circuit 33a is designed to match, even when the impedance of the load is Z0, the impedance Z0 of the load and the output impedance of carrier amplifier 31a with each other. In this case, since carrier and peak amplifiers 31a and 31b both output power Pmax, the output of the entire Doherty-type power amplifier is 2Pmax. Further, carrier and peak amplifiers 31a and 31b both operate in saturation, and accordingly the efficiency of the entire power amplifier is 78.5%. In other words, in the Doherty-type power amplifier, since power efficiency when the amplitude of the output signal is ½·Vmax (corresponding to 6 dB back-off in the case of power) is improved, power consumption at a low output can be reduced.
Concerning the recent wireless communication standard, to achieve much faster wireless communication, a study has been conducted on CA (Carrier Aggregation) technology that uses a plurality of fragmented (discontinuous) frequency bands for transferring one signal. The CA technology is described in, for example, Non-patent Document 2.
In the wireless communication system employing the CA technology, since the frequency bands usable for data transmission increases, a much higher transmission speed can be achieved.
In the wireless communication system employing the CA technology, in the case of frequency allocation where carrier frequencies are greatly separated from each other (when difference Δf between carrier frequencies is sufficiently larger than modulation band width fBB of each carrier frequency: inter-band non-contiguous CA), communication is carried out by simultaneously using a plurality of carrier frequencies having different transmission characteristics, and thus communication stability can be improved.
Further, the application of the CA technology enables communication even when frequency bands are allocated in fragments to a plurality of business operators or when a frequency band is shared among the plurality of business operators.
In the wireless communication system employing the aforementioned CA technology, a transmission device for transmitting a RF signal of a plurality of frequency bands (bands) is necessary. Even such a transmission device is required to reduce power consumption.
Typically, the characteristics of the power amplifier strongly depend on the frequency of the RF signal to be amplified. Accordingly, to achieve a transmission device compatible with the aforementioned CA technology, a configuration where a dedicated power amplifier is provided for each used frequency band is generally employed.
FIG. 3 illustrates the configuration example of a transmission device which uses two frequencies f1 and f2 as carrier frequencies and to which the CA technology is applied according to the background art.
The transmission device illustrated in FIG. 3 is configured by including first power amplifier 41 for amplifying and outputting RF signal 46a of the carrier frequency f1, second power amplifier 42 for amplifying and outputting RF signal 46b of the carrier frequency f2, combiner 44 for combining the output signals of first and second power amplifiers 41 and 42, and broadband antenna 45 corresponding to the carrier frequencies f1 and f2.
For first and second power amplifiers 41 and 42 illustrated in FIG. 3, to reduce the power consumption of the entire transmission device, the power amplifier of the ET system illustrated in FIG. 1 or the Doherty-type power amplifier illustrated in FIG. 2 is preferably used.
As described above, the characteristics of the power amplifier used in the transmission device exhibit strong frequency dependence. Accordingly, to reduce the power consumption while realizing a transmission device compatible with the aforementioned CA technology, a configuration where a dedicated power amplifier is provided for each used frequency band is generally employed as illustrated in FIG. 3.
However, such a transmission device has a problem of a size increase due to the necessity of including a plurality of power amplifiers.
In the transmission device of the background art illustrated in FIG. 3, when the used carrier frequency is changed, a new power amplifier optimally designed to correspond to the changed carrier frequency must be provided. This creates a problem of inability to flexibly deal with the carrier frequency change.